In a vehicle air bag system, an inflator (or gas generator) is actuated during a collision, and generates gas which inflates an air bag. While various types of inflator structures are currently in use, virtually all inflators have the following basic structures: (i) a container having gas discharge nozzles and a solid gas generating composition, (ii) a filter disposed in the container between the solid gas generating composition and the gas discharge nozzles of the container, and (iii) an ignition device for igniting the gas generating composition.
At the onset of a collision, the ignition device is detonated, and causes ignition of the solid gas generating composition. Burning of the composition generates large quantities of gas. The gas is directed under pressure through the gas discharge nozzles and into the air bag to inflate the air bag. The air bag is inflated within a time frame sufficient to prevent a vehicle occupant from impacting part of the vehicle.
In any air bag system, it is essential that the system deploy the air bag within a specified time of sensing the onset of a collision. The ability of the system to deploy the air bag within the specified time depends to a significant extent on the performance characteristics of the inflator within that time, i.e. the ability of the inflator to generate significant quantities of gas and direct the gas into the air bag within the specified time.
Over the years, different inflator structures with different gas generating compositions having burning surfaces with different geometries have been suggested. The chemicals used in different gas generating compositions and the physical form of the compositions (e.g., loose powder, pressed tablets, or complex, perforated grains) also produce different inflator performance characteristics. For example, certain gas generating compositions, generally referred to as regressive burn gas generating compositions, have an initial burn during which the burning surface is very large, and gas is generated very rapidly. After the initial burn, the rate of gas generation rapidly declines as the burning surface of the composition declines. Other compositions, referred to as progressive and neutral burn gas generating compositions, have smaller burning surfaces for a slower initial burn than regressive burn gas generating compositions, and thus generate gas at a slower rate. With progressive burn gas generating compositions, the rate of gas generation increases to an initial rate. Once the initial rate of burning is achieved, the burning surface, and the rate of gas generation, continue to increase until a peak rate is reached. After reaching the peak rate, the burning surface and gas generation rate decline. With neutral burn gas generating compositions, gas is initially generated at a peak rate. The burning surface is constant after the peak rate is achieved, and gas continues to be generated at the constant rate, until the burning surface and gas generation rate decline.
The performance characteristics of an inflator are also affected by the manner in which a gas generating composition is stored in the inflator, and the manner in which gas flow is directed and filtered before leaving the inflator. For example, in a container for gas generating compositions illustrated in U.S. Pat. No. 4,414,902, a liner of laminated plastic and metal is heat and pressure sealed to a metal blank, which is rolled to form a gas generating container having a hermetic liner, for insertion into an inflator unit. Further, in an inflator illustrated in U.S. Pat. No. 4,296,084, a gas generating composition and a primary filter are disposed within a sealed cartridge contained in an inflator. The cartridge is pre-formed, loaded with the gas generating composition and the primary filter, sealed, and then assembled with the inflator. The cartridge protects the gas generating composition and the filter from the ambient environment. During operation of the inflator, the cartridge ruptures, and gas generated within the cartridge flows through a secondary filtering and cooling structure before flowing out of the inflator.
In an inflator illustrated in U.S. Pat. No. 4,878,690, a gas generating composition and a primary filter device are loaded within a special barrier tube inside the inflator. The barrier tube has openings facing away from the gas discharge nozzles of the inflator. A rupturable membrane is disposed inside the barrier tube to cover the openings in the barrier tube. A pair of end caps close the barrier tube and seal the gas generating composition and the filter in the tube. Another filter surrounds the barrier tube. When the inflator is actuated, the membrane ruptures, and directs gas flow away from the gas discharge nozzles. Thus, the gas flow is along a path through the filter surrounding the barrier tube, before the gas flows out of the inflator.
It is believed that with current gas generating compositions and filter materials, it is important to seal a gas generating composition and filter hermetically within an inflator. Moreover, applicant believes there is a need to enhance the performance characteristics of inflators using progressive and/or neutral burn gas generating compositions. Additionally, applicant believes there is a need to provide such features with an inflator which is relatively simple in design, and efficient to assemble.